The IBM System/370 "Principles of Operation" (Form No. 22-7000-4), page 38, defines the key-in-storage architecture which associates a key-in-storage byte with each 2048 byte (i.e. 2K) block in real main storage. The key-in-storage byte includes a four bit access control key, a fetch control bit F, a reference bit R, and a change bit C. An executing program is only permitted to access data in main storage when the program has in its program status word (PSW) the key assigned to the block to be accessed for the requested data, or if the program has a key equal to zero which is a master key that can access any block in storage and is generally used by the system control programs. If the fetch-protection bit F is set, the associated block is not to be fetched or stored without a key match or zero key. The reference bit R is set whenever a fetch or store access is made to the block, and the change bit C is set whenever the block is stored into. Special instructions are provided for setting and resetting values into these different fields in the key-in-storage byte.